1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic isomerization of olefinic hydrocarbons.
2. General Background Olefinic hydrocarbons are feedstocks for a variety of commercially important addition reactions to yield fuels, polymers, oxygenates and other chemical products. The specific olefin isomer, considering the position of the double bond or the degree of branching of the hydrocarbon, may be important to the efficiency of the chemical reaction or to the properties of the product. The distribution of isomers in a mixture of olefinic hydrocarbons is rarely optimum for a specific application. It is often desirable to isomerize olefins to increase the output of the desired isomer.
Butenes are among the most useful of the olefinic hydrocarbons having more than one isomer. A high-octane gasoline component is produced from a mixture of butenes in many petroleum refineries principally by alkylation with isobutane; 2-butenes (cis- and trans-) generally are the most desirable isomers for this application. Secondary-butyl alcohol and methylethyl ketone, as well as butadiene, are other important derivatives of 2-butenes. Demand for 1-butene has been growing rapidly based on its use as a comonomer for linear low-density polyethylene and as a monomer in polybutene production. Isobutene finds application in such products as methyl methacrylate, polyisobutene and butyl rubber. The most important derivative influencing isobutene demand and butene isomer requirements, however, is methyl t-butyl ether (MTBE) which is experiencing rapid growth in demand as a gasoline component.
Pentenes also are valuable olefinic feedstocks for fuel and chemical products. Isoprene, which may be produced by dehydrogenation of isopentene, is an important monomer in the production of elastomers. To an increasing extent, pentenes obtained from refinery cracking units are alkylated with isobutane to obtain a high-octane gasoline component. The principal influence on trends in isopentene demand and pentene isomer requirements, however, is the rapid growth in demand for methyl t-amyl ether (TAME) as a gasoline component. This derivative is of increasing interest as restrictions on gasoline olefins and volatility reduce the utility of pentenes as a gasoline component and as ethers and alcohols are needed for reformulated gasolines with higher oxygen content. This interest may extend to hexenes and higher olefins having tertiary carbons which could be reacted to yield high-octane ethers.
Olefin isomers rarely are obtained in a refinery or petrochemical product in a ratio matching product demand. In particular, there is a widespread need to increase the proportion of isobutene, isopentene and other tertiary-carbon olefins for production of MTBE, TAME and other ethers. Catalytic isomerization to alter the ratio of isomers is one solution to this need. Since ethers must be supplied at lower cost to find widespread use as a fuel product and since isomerization competes with increased feedstock processing as a source of desired isomers, an isomerization process must be efficient and relatively inexpensive. In one aspect, a catalytic isomerization process must recognize olefin reactivity: isobutene in particular readily forms oligomers which could require a reconversion step to yield monomer if produced in excess. The principal problem facing workers in the art therefore is to isomerize olefins to increase the concentration of the desired isomer while minimizing product losses to heavier or lighter products.
3. Related Art
Processes for the isomerization of olefinic hydrocarbons are widely known in the art. Many of these use catalysts comprising phosphate. U.S. Pat. No. 2,537,283 (Schaad), for example, teaches an isomerization process using an ammonium phosphate catalyst and discloses examples of butene and pentene isomerization. U.S. Pat. No. 3,211,801 (Holm et al.) discloses a method of preparing a catalyst comprising precipitated aluminum phosphate within a silica gel network and the use of this catalyst in the isomerization of butene-1 to butene-2. U.S. Pat. Nos. 3,270,085 and 3,327,014 (Noddings et al.) teach an olefin isomerization process using a chromium-nickel phosphate catalyst, effective for isomerizing 1-butene and higher alpha-olefins. U.S. Pat. No. 3,304,343 (Mitsutani) reveals a process for double-bond transfer based on a catalyst of solid phosphoric acid on silica, and demonstrates effective results in isomerizing 1-butene to 2-butenes. U.S. Pat. No. 3,448,164 (Holm et al.) teaches skeletal isomerization of olefins to yield branched isomers using a catalyst containing aluminum phosphate and titanium compounds. U.S. Pat. No. 4,593,146 teaches isomerization of an aliphatic olefin, preferably 1-butene, with a catalyst consisting essentially of chromium and amorphous aluminum phosphate. None of the above references disclose the olefin-isomerization process using the non-zeolitic molecular sieve (NZMS) of the present invention.
The art also contains references to the related use of zeolitic molecular sieves. U.S. Pat. No. 3,723,564 (Tidwell et al.) teaches the isomerization of 1butene to 2-butene using a zeolitic molecular sieve. U.S. Pat. No. 3,751,502 (Hayes et al.) discloses the isomerization of mono-olefins based on a catalyst comprising crystalline aluminosilicate in an alumina carrier with platinum-group and Group IV-A metallic components. U.S. Pat. No. 3,800,003 (Sobel) discloses the employment of a zeolite catalyst for butene isomerization. U.S. Pat. No. 3,972,832 (Butler et al.) teaches the use of a phosphorus-containing zeolite, in which the phosphorus has not been substituted for silicon or aluminum in the framework, for butene conversion. None of the above teach the use of NZMS for selective butene isomerization, and Butler et al. discloses high yields of heavier olefins from butenes at a range of temperatures with a phosphorus-containing zeolite.
U.S. Pat. No. 4,503,282 (Sikkenga) reveals a process for converting linear alkenes to isomerized alkenes using a crystalline borosilicate molecular sieve, with examples demonstrating the conversion of linear butenes to isobutene. U.S. Pat. No. 5, 132,467 (Haag et al.), filed Mar. 6, 1991, teaches a combination of two-stage etherification followed by common fractionation and olefin isomerization; the isomerization is carried out over a medium-pore metallosilicate catalyst with a range of ZSMs and MCM-22 being disclosed. The isomerization of olefins using NZMS, containing tetrahedral aluminum, phosphorus and at least one other element, has not been disclosed in the above references.
U.S. Pat. No. 5,107,050 (Gaffney et al.), filed Dec. 28, 1990, discloses butene isomerization using a MgAPSO or SAPO molecular sieve at a temperature above 900.degree. F. U.S. Pat. No. 5,136,108 (Gaffney et al.), filed Mar. 6, 1991, teaches a combination process for producing TAME and/or TAA by reacting tertiary pentenes with methanol and/or water, distillation to separate reactants, and isomerization of C.sub.5 hydrocarbons with return of branched hydrocarbons to TAME/TAA production; preferred isomerization catalysts are SAPOs and MgAPSOs.
"Non-zeolitic molecular sieves" or "NZMSs" as referenced herein include the "SAPO" silicoaluminophosphates of U.S. Pat. No. 4,440,871 (Lok et al.), the "FAPO" ferroaluminophosphates of U.S. Pat. No. 4,554,143 (Messina et al.), and the metal aluminophosphates of U.S. Pat. No. 4,567,029 (Wilson et al.) wherein the metal is at least one of Mn, Co, Zn and Mg. The application of NZMS-containing catalyst to the isomerization of a C.sub.8 aromatics stream is revealed in U.S. Pat. No. 4,740,650 (Pellet et al.). U.S. Pat. No. 4,689,138 teaches a process for isomerizing normal and slightly branched paraffins using a catalyst comprising SAPO molecular sieves. The use of MgAPSO compositions for hydrocarbon conversion is taught in U.S. Pat. No. 4,882,038. However, none of these references discloses or suggests the isomerization of olefins using a catalyst containing NZMS and having the absence of a hydrogenation promoter.